The present invention relates to a method of dispersing carbon nanotubes (CNTs) in a continuous phase, especially in at least one dispersion medium, and also to the dispersions themselves that are obtainable in this way, and to their use.
Carbon nanotubes (CNTs for carbon nanotubes) are microscopically small tubular structures (i.e., molecular nanotubes) of carbon. Their walls—like those of the fullerenes or like the planes of graphite—are composed essentially exclusively of carbon, the carbon atoms occupying a honeycomblike structure with hexagons and with three bond partners in each case, this structure being dictated by the sp2 hybridization of the carbon atoms.
Carbon nanotubes derive accordingly from the carbon planes of graphite which have, so to speak, been rolled up to form a tube: The carbon atoms form a honeycomblike hexagonal structure and three bond partners in each case. Tubes with an ideally hexagonal structure have a uniform thickness and are linear; also possible, however, are bent or narrowing tubes which contain pentagonal carbon rings. Depending on the way in which the honeycomb network of the graphite is rolled to form a tube (“straight” or “oblique”), there are helical (i.e., wound in the manner of a screw) and also nonmirror-symmetric structures, i.e., chiral structures.
A distinction is made between single-wall carbon nanotubes (SWCNTs or SWNTs) and multiwall carbon nanotubes (MWCNTs or MWNTs), between open or closed carbon nanotubes (i.e., with a “cap”, which for example is a section of a fullerene structure), and also between empty and filled (with, for example, silver, liquid lead, noble gases, etc.) carbon nanotubes.
The diameter of the carbon nanotubes (CNTs) is in the region of a few nanometers (e.g., 1 to 50 nm), although carbon nanotubes (CNTs) with tube diameters of only 0.4 nm have already been prepared. Lengths ranging from several micrometers up to millimeters for individual tubes and up to a few centimeters for tube bundles have already been attained.
Depending on the detail of the structure, the electrical conductivity within the carbon nanotubes is metallic or semiconducting. There are also carbon nanotubes known which at low temperatures are superconducting.
Transistors and simple circuits with semiconducting carbon nanotubes have already been produced. Additionally, attempts have already been made to carry out specific production of complex circuits from different carbon nanotubes.
The mechanical properties of carbon nanotubes are outstanding: CNTs have—for a density of, for example, 1.3 to 1.4 g/cm3—an enormous tensile strength of several megapascals; in comparison to this, steel, for a density of at least 7.8 g/cm3, has a maximum tensile strength of only about 2 MPa, thus giving a ratio of tensile strength to density, arithmetically, which for some CNTs is at least 135 times better than for steel.
Of particular interest for the field of electronics are the current rating and the electrical and thermal conductivities: The current rating is by estimation 1000 times higher than in the case of copper wires, whereas the thermal conductivity at room temperature is almost twice as high as that of diamond. Since CNTs may also be semiconductors, they can be used to manufacture outstanding transistors which withstand higher voltages and temperatures—and hence higher clock frequencies—than silicon transistors; functioning transistors have already been produced from CNTs. Furthermore, CNTs can be used to realize nonvolatile memories. CNTs can also be used in the field of metrology (e.g., scanning tunneling microscopes).
On the basis of their mechanical and electrical properties, carbon nanotubes can also find application in plastics: As a result, for example, the mechanical properties of the plastics are greatly improved. Furthermore, it is possible in this way to produce electrically conducting plastics.
Carbon nanotubes (CNTs) are commercially available and are supplied by numerous manufacturers (e.g., by Bayer MaterialScience AG, Germany; CNT Co. Ltd., China; Cheap Tubes Inc., USA; and Nanocyl S. A., Belgium). Corresponding manufacturing processes are familiar to the skilled worker. Thus, for example, carbon nanotubes (CNTs) can be prepared by arc discharge between carbon electrodes, for example; by means of laser ablation (“vaporization”) starting from graphite; or by catalytic decomposition of hydrocarbons (chemical vapor deposition, CVD for short).
The properties described above for the carbon nanotubes (CNTs) and the possible applications which arise from these properties have awoken great interest. In particular, for a range of applications, there is a need for the carbon nanotubes (CNTs) to be provided in a readily manageable form, preferably in the form of dispersions.
The dispersing of carbon nanotubes (CNTs) poses a great challenge, since the carbon nanotubes (CNTs) are very difficult to convert into stable dispersions, especially because the carbon nanotubes (CNTs) possess a very high aspect ratio and are present in highly agglomerated and/or coiled forms.
In the prior art, therefore, there has been no lack of attempts to stably disperse carbon nanotubes (CNTs). The methods known from the prior art, however, are not very suitable for generating stable, concentrated dispersions of carbon nanotubes (CNTs): in the majority of cases the methods of the prior art do not lead to storage-stable dispersions, and, moreover, the concentration of carbon nanotubes (CNTs) in the prior-art dispersions is usually extremely small.
Thus, certain prior-art methods are aimed first at modifying the surface of the carbon nanotubes (CNTs) to be dispersed, by means of a costly and inconvenient prior pretreatment, particularly for the purpose of making the surface polar in order to facilitate subsequent dispersing. Methods suitable for modifying the carbon nanotubes (CNTs) are, for example, oxidative processes, especially chemical pretreatment, halogenation, or other polarization processes for modifying the surfaces of the carbon nanotubes (CNTs). One method of this kind, which provides, for example, for prior fluorination of the surfaces of the carbon nanotubes (CNTs) prior to their dispersing, is described for example in U.S. Pat. No. 6,827,918 B2.
A disadvantage of these methods is the costly and inconvenient pretreatment, which particularly when implemented on an industrial scale results in more difficult implementation of the method, with significantly higher costs.
Also described in the prior art have been methods which convert the carbon nanotubes into aqueous dispersions in the presence of a water-soluble polymer material (cf., e.g., US 2004/0131859 A1 and WO 02/076888 A1). These methods, however, have the disadvantage that, on the one hand, they are not universally employable, but instead are restricted to aqueous dispersion media, and, on the other hand, that they lead only to dispersions having relatively low carbon nanotube (CNT) contents. The prevailing object of the two aforementioned publications, instead, is the conversion of the dispersions described therein into a redispersible powder of carbon nanotubes (CNTs).
The above-described methods of the prior art usually result in inhomogeneous dispersions, often lacking long-term stability, of carbon nanotubes (CNTs), with low concentrations or contents of CNTs. Furthermore, the prior-art dispersions—in comparison to the pure dispersion medium—exhibit a high to extreme increase in viscosity, coupled with low particle contents of carbon nanotubes (CNTs), of only up to about 1% by weight.
In relation to industrial implementation, these prior-art dispersions are associated with a great disadvantage, and consequently there is an increased demand for improved dispersions of carbon nanotubes (CNTs) in various media.